To determine the sensitivity of a magnetic field sensor, a magnetic field having a magnetic flux density, which should be known, if possible, is created at the location of the sensor by impressing a defined current into a coil or an exciting (excitation) conductor. The resulting change in the output signal of the magnetic field sensor upon application of a calibrating magnetic field may then be used to infer the sensitivity of the sensor. Thus, in a Hall sensor element, a change in the Hall voltage, which is caused by a change in the magnetic flux density in the sensor element, may be used to infer the actual sensitivity of the Hall sensor element.
With exciting (excitation) coils or exciting conductor patterns, the problem may arise that during manufacturing of the magnetic field sensor in a semiconductor substrate, the individual layer structures are subjected to process variations as typically occur in the manufacturing of semiconductor devices. In a semiconductor device, said process variations are generally considerably larger in the vertical direction, i.e. in a direction perpendicular to the substrate surface, than in a lateral direction, i.e. in a plane parallel to the semiconductor substrate surface. Accordingly, an actual value of a distance between the active area of the Hall sensor element and the exciting conductor pattern may deviate from an ideal distance value aimed at during manufacturing. Since in the calibration of the magnetic field sensor by means of a magnetic field created by a defined calibration current, the position and/or the effective distance of the exciting line from the active area of the Hall sensor element is accounted for, any process tolerances not taken into account may lead to inaccurate calibration of the magnetic field sensor.
FIGS. 6a-b show a schematic diagram in the form of a top view and a sectional view of a vertical Hall sensor element 100 in a semiconductor substrate 108. Vertical is supposed to mean a plane perpendicular to the semiconductor substrate and/or chip surface 108a, i.e. vertical to the x-z plane in FIGS. 6a-b. The vertical Hall sensor element 100 shown in FIGS. 6a-b comprises five contact areas 102a-e along the main surface of the active semiconductor area 104. Vertical Hall sensors, which may sense a magnetic field component B0x in the active area parallel to the substrate surface (in the x direction), may be provided with a calibration flux density {right arrow over (B)}0 in a targeted manner for calibration by means of an exciting conductor 106 through which a calibration current I0 flows. As is schematically shown in FIG. 6a, the exciting conductor 106 is arranged at a height h0 directly above, or perpendicularly above, the Hall sensor element 100. As is further depicted in FIGS. 6a-b, the active area 104 of the Hall sensor element 100 has a width s0, for example, while the exciting conductor 106 has a width e0.
In this manner, the magnetic flux density B0x created at the location of the sensor by the exciting conductor 106 may be approximately specified on the basis of the distance h0 between the Hall sensor element 100 and the exciting conductor 106, the sensor width s0, the exciting conductor width e0 and the current I0 flowing through the exciting conductor 106.
In a vertical Hall sensor element, the above-mentioned process tolerances may have a particularly strong impact since the distance h0 between the active area 104 of the Hall sensor element 100 and the exciting conductor 106 may frequently vary within a range of ±40% of the actual target distance h0 due to process variations or process tolerances. As a result, the sensitivity of vertical Hall sensor elements can be determined with a relatively low level of accuracy only.
FIG. 7 shows, e.g., the dependence of the magnetic flux density B0x (e.g. in μT) on the height h0 (in μm) of the exciting conductor 106 above the active area 104 of the Hall sensor element 100. The magnetic flux density B0x decreases in proportion to 1/h0 over the distance h0. If one assumes, for example, a distance h0 of 5 μm, within a tolerance range of, e.g., only about ±20% (±1 μm) of the distance h0, marked differences from the expected values for the magnetic flux density B0x will result in the active area 104 of the Hall sensor element 100. Thus, already relatively small changes Δh0 in the distance h0 result in relatively marked changes in the magnetic flux density B0x created in the active area 104 of the Hall sensor element 100.
FIG. 8 shows the change in the magnetic flux density {right arrow over (β)}0 at the location of the sensor, i.e. in the active area of the Hall sensor element 100, in dependence on the process tolerances Δh0 with regard to the distance h0. If one assumes, again, a target distance h0 of 5 μm, the magnetic flux density B0x which is expected for a predefined calibration current intensity I0 will amount to 33 μT, for example. However, if the distance h0 varies by about Δh0=±20% (e.g. ±1 μm) due to the process tolerances, for example, a distance h0 of 4 mm will result in a magnetic flux density of about 38 μT, whereas a distance h0 of 6 μm will result in a magnetic flux density of about 28 μT. Thus, the resulting magnetic flux density B0x will vary already by more than 25% if the distance h0 is exposed only to a tolerance range of Δh0=±20% due to process variations. Actually, process variations of ±40% may frequently occur for layer thicknesses in semiconductor manufacturing processes. As a result, precise determination of the sensitivity of the Hall sensor element 100 will at least be very imprecise if the calibration is performed only with an assumed exciting conductor distance h0, which in reality is often not fully correct, or imprecise, due to the process tolerances Δh0, rather than being performed on the basis of a knowledge of the actual distance value h0±Δh0.